Incomplete ideas, thoughts and jottings
Store every failed poem, writing line and idea in one big file to sort through later. Comb through it regularly when unispired and see what jumps out at you. Similiar to a writers notebook, but digital and searchable.
Ideas compost. The spark file is the compost pile where things break down and combine in new ways.
I've kept a spark file for years and it's many, many lines long. Because it's plain text, it's still a compact file and able to be easily searched and looked through. I have it stored as an org file in emacs and have a function to show a random headline.
It's useful to look through years later and see the start of ideas.